1. Field of the Invention
The present invention relates to an industrial robot, and more particularly to a wrist structure to be fitted to a tip of a robot forearm.
2. Description of the Prior Art
FIG. 7A and FIG. 7B illustrate a state of piping and wiring for supplying electric power, oil and air to, and discharging exhaust gas from, a spot welding gun when spot welding is to be performed by an industrial robot. Referring to FIG. 7A and FIG. 7B, a wrist 20 is fitted to a tip of a robot forearm 10. The wrist 20 is provided with swivel shafts 21, 22 and 23, differing in an axial direction of swivel from and linked to one another. The swivel shaft 23 is fitted with a spot welding gun 5 as an end effector. Also, the wrist 20 is fitted at the other end to the robot forearm 10 rotatably around the swivel shaft 21.
Further, a midway portion of piping and/or wiring member (hereinafter referred to as piping/wiring member) 4 is held by a clamp 7 fixed to the robot forearm 10, and the tip of the same member is held by another clamp 6 fixed to the spot welding gun 5.
A method for letting piping/wiring member, including a tube for flow of a pressure transmitting medium, such as hydraulic oil, and a cable for feeding of an electric power, pass through a robot forearm is also known, as disclosed in EPO 873826A2 for instance.
Where the piping/wiring member 4 is arranged outside the robot as shown in FIG. 7A, the piping/wiring member 4 is given an extra length, not so great as to obstruct the actions of wrist shafts, and connected to the gun 5. However, this method has disadvantages in that the piping and wiring come into contact with the robot arm or peripheral units and that, as shown in FIG. 7B, the piping and wiring would easily get twined round the wrist. Moreover, as a consequence of this extra length, the piping/wiring member 4 may hang down from the forearm and the wrist and, because the behavior of this hanging part is unpredictable, there arises a problem that the work procedure has to be taught to robots one by one, each requiring checkup of the hanging state.
A method of building the piping/wiring member 4 into the robot arm, disclosed in EPO 873826A2 mentioned above, involves the problem that, because of the complexity of the wrist structure, maintenance work on the wrist is troublesome and the locus is difficult to be kept accurate.
An object of the present invention is to provide a wrist structure that can prevent a piping/wiring member from getting twined round the robot wrist, coming into contact with elements of the wrist and being damaged by scratching.
In order to achieve the object stated above, a wrist according to the present invention comprises first, second and third wrist elements. Where the robot is so structured that a forearms is provided rotatably around a first axis, which is its lengthwise axis, the first wrist element is integrally fitted to the tip of this forearm. Where the forearm of the robot does not rotate, the first wrist element is fitted rotatably to the forearm. The second wrist element is provided rotatably around a second axis which crosses the first axis at a prescribed angle to the first wrist element, and the third wrist element having a hollow path is provided rotatably around a third axis which crosses the second axis at a prescribed angle to the second wrist element. Further a first transmission element on the second axis, which is one element of a drive power transmission mechanism for transmitting power to drive the second wrist element relative to the first wrist element, and a second transmission element on the second axis, which is one element of a drive power transmission mechanism for transmitting power to drive the third wrist element relative to the second wrist element, are positioned offset by a prescribed distance in the same direction relative to the third axis along the second axis. The central axis of rotation of a drive power transmission element for directly transmitting drive power to the first transmission element is arranged in parallel to the second axis, and a space for wiring and piping is provided near the point of intersection between the second axis and the third axis.
The wiring or piping to the end effector may be laid along the first wrist element, and connected to the end effector via the hollow path. Part or the whole of the first through third wrist elements and the hollow path may be provided with protective devices for reducing scratching of the wiring or piping to the end effector. Part or whole of the span from the first wrist element to the hollow path may be provided with accommodating members for accommodating the wiring or piping of the end effector.
Since the span of the piping/wiring member from the first wrist element to the third wrist element is made a void area according to the present invention, the piping/wiring member shifts in this void area and absorbs any variation in length even if the wrist elements rotate, there is no possibility for the piping/wiring member to twine round the wrist, and the parts of the piping/wiring member in sliding friction with elements of the wrist and peripheral units are reduced, resulting in reducing of damage to piping/wiring members by scratching.